Williamson's writing career has its roots in his obligatory national service as a young man. While in the Royal Air Force, he began a life-long association with fellow airman, Dennis Spooner, by writing some amateur efforts. After the war, he accepted a position as a news correspondent for CBS in Canada. On the side, he wrote at least twenty stage plays that were later produced on various anthological television programmes.

When he returned to Britain, he flirted with soap operas like Coronation Street and Compact. However, by the mid-1960s, he embarked upon his career of writing spy fiction with the sale of a script to The Mask of Janus. Though he also contributed to its spin-off, The Spies, Williamson swiftly moved on to a more fantastic espionage setting with his first sale to The Avengers in 1965. He wrote a few more scripts for the Emma Peel era before being tapped by Sydney Newman to script edit Adam Adamant Lives! When Newman cancelled that show, he returned to The Avengers, for which he was a dominant writer of the show's Peel-less final season.

Following the demise of The Avengers, he worked on a number of programmes on which Dennis Spooner held some measure of creative control as creator or story editor, for ITC Entertainment. Projects like Randall and Hopkirk (Deceased), The Champions, and Department S all came to accept his script submissions following Adamant!. His initial sale to Department S was particularly significant in that it began a series of sales involving the character of Jason King, the lead in two programmes in the early 1970s. His final script using the spy, Jason King's "A Page Before Dying", would mark the passage of Williamson's longest association with a single fictional character.

Aside from his work in television, Williamson wrote several Fontana published thrillers in the 1970s under his own name and at least one ('Slade's Marauder' Souvenir Press, 1980) under the pseudonym Steven Cade. At the same time he wrote for the big screen. 'Night Watch' (1973) featured a Williamson screenplay and a cast including Elizabeth Taylor, Billie Whitelaw and Laurence Harvey. The less well reviewed 'Breakthrough' (1979) co-written with Peter Bernies starred Richard Burton and Rod Steiger. The film came heavily indebted to Sam Peckinpah's classic 'Cross of Iron'.

After heart surgery in 1980, Williamson deliberately slowed his output. However, he was the dominant contributor to the 1986 supernatural series, Worlds Beyond.

Williamson died during a second heart procedure in 1991, soon after his third film, a minor Roger Moore vehicle called Fire, Ice & Dynamite, opened in West Germany.